Why aren't my text messages sending out?

I have Sprint and a Rumor Touch.

I know I should call them but I thought I’d ask here first.

I’ve texted this person many times before and each time they’ve received my texts but for the past three or four days I have not been able to send any out to them. But they can still send ME texts.

Any reason why my texts aren’t sending out? In the beginning it said “Error 9230. Destination email address is not in the correct format.”? What does this mean?
I’m pretty I wasn’t sending an email to them. I also went online to my account to make sure I hadn’t blocked any texts.
Anyone have any idea what’s going on?